Freeze with args

[megies]

When I use pip freeze I am usually (95% of all times) interested to find out what version of one specific module is installed, rather than listing all packages at their current version. So, I end up doing something like e.g. pip freeze | grep nump all the time to find out what numpy version is installed. I propose to allow simply saying something like pip freeze num sci to only show versions of modules that match the given search strings.

[msabramo]

#1946 also does this, and a bunch of other stuff.

[AvdN]

This seems to do exactly what I proposed in Mar 2013 extended with some tests

[msabramo]

These all have merge conflicts:

• arguments to freeze are used to filter installed packages #858
• Freeze with args #1308
• 'pip freeze PKG' outputs the installed version of PKG and its dependencies #1946

#2234 is #1946 but rebased so it doesn't have merge conflicts now, but I don't know how long that is going to last.

[megies]

#1946 also does this, and a bunch of other stuff.

Like I already stated over there at #1946.. it looks more advanced and I am not offended at all if this PR here gets closed unmerged in favor of another PR.

Edit: I'm gonna close this one I think. Feel free to reopen if necessary (which I doubt).